Inkjet printers have printheads that include a plurality of inkjets, each of which ejects liquid ink onto an image receiving member as the member moves past the printhead. The ink may be stored in reservoirs fluidly connected to the printheads or located within cartridges installed in the printer. Various forms of ink include aqueous, oil, solvent-based, UV curable inks, or ink emulsions. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink may be pellets, ink sticks, granules, pastilles, or other forms. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device that melts the ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. In other inkjet printers, ink may be supplied in a gel form. The gel is also heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead.
A typical full width inkjet printer uses one or more printheads to eject a line of ink across a width of an image receiving member. Each printhead typically contains an array of inkjets configured to eject drops of ink across an open gap to the image receiving member. The image receiving member may be a continuous web of recording media, a series of media sheets, or the image receiving member may be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force generated in a transfix nip that is formed by the rotating surface and a transfix roller. In an inkjet printhead, each inkjet includes an individual piezoelectric, thermal, or acoustic actuator that generates mechanical forces that expel ink from a pressure chamber through an orifice in response to an electrical signal, also referred to as a driving or firing signal. The amplitudes, or voltage levels, of the signals affect the amount of ink ejected in each drop. The firing signal is generated by a printhead controller in accordance with image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops land are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
Over the operational life of an inkjet printer, the actuators within the printheads that eject the ink from the inkjets often degrade. Many factors contribute to this degradation including age of the printhead, number of inkjet actuations, temperature of the printhead, etc. The temperature-related degradation arising from thermal effects is commonly known as printhead thermal drift. In some printers, the performance of some of the inkjets in one or more printheads begins to degrade. Typically, this inkjet degradation is exemplified in reduced ink drop mass and variation in the placement of the ejected ink drops on the image receiving member. Inkjets that operate with these performance deviations are typically known as weak inkjets. Eventually, the resulting ink drop density and faulty ink drop placement affects image quality. Existing printing systems attempt to compensate for this aging or thermal effect on inkjets using a predetermined aging function that adjusts the electrical firing signal supplied to the inkjets based on various factors, including the temperature of the printhead and the amount of time that the printhead is operated at the temperature. The existing techniques have limitations because individual printheads are affected by temperature differently over the life of a printhead, and the predetermined aging functions do not reliably predict operational changes in each printhead in a printer. Consequently, improved methods and systems for identifying the firing signal parameters that optimally operate printheads in a printer to reduce the number of weak and/or inoperable inkjets would be beneficial.